An antilock braking system (commonly known as ABS) is a system used on a wheeled vehicle which prevents the wheels from locking when brakes acting on each of the wheels are applied. A typical ABS comprises a control module, wheel speed sensors and one or more brake hydraulic modulators. A wheel speed sensor is typically associated with each wheel (alternatively, a pair of wheels on a common axle can share a wheel speed sensor). The wheel speed sensor provides a speed signal to the control module from which the control module can derive the speed of the wheel as well as acceleration and deceleration of the wheel. Based on the speed signals received from the wheel speed sensors and the application of one or more control algorithms, the control module can determine when one of the wheels is approaching lock-up. When a wheel is approaching lock-up the control module sends a control signal to the hydraulic modulator to modulate the brake acting on the wheel. The hydraulic modulator modulates the brake by alternately relieving and reapplying the brake pressure applied by a brake master cylinder to the brake at the wheel.
The hydraulic modulator typically comprises solenoid operated valves connected to a brake circuit associated with each wheel. The valves permit hydraulic pressure applied to the brake by the master cylinder to be interrupted and the pressure relieved and reapplied. In some implementations the hydraulic modulator can also comprise a high-pressure hydraulic pump and a pressure accumulator. The pump and accumulator can be used to reapply pressure to the brake as the hydraulic modulator cycles between relieving and reapplying pressure to the brake. The hydraulic modulator is typically a complex and expensive-to-manufacture component that increases the cost of producing and operating a vehicle equipped with a conventional antilock brake system.